The polyolefins and polyvinyl chlorides can be considered to be the two major families of plastic resins from which the bulk of commercially available shrink films for wrapping purposes are made. Other resin families from which shrink films can be made include the ionomers, polyesters, polystyrenes and polyvinylidene chlorides. The shrinkable polyolefins currently on the market are mainly monolayer films which include both cross-linked and uncross-linked oriented polyethylene, oriented polypropylene, and oriented ethylene-propylene copolymers. The polyvinyl chloride (hereinafter "PVC") shrink films ar monolayer films consisting of a variety of formulations of polyvinyl chloride.
A shrink film's distinguishing characteristic is its ability upon exposure to some level of heat to shrink or, if restrained, to create shrink tension within the film. This ability is activated by the packager when the wrapped product is passed through a hot air or hot water shrink tunnel. This process causes the film to shrink around the product producing a tight, transparent wrapping that conforms to the contour of the product and which is aesthetically pleasing while providing the useful functions required of packaging materials such as protection of the product from loss of components, pilferage, damage due to handling and shipment, dirt and contamination. Typical items wrapped in PVC or polyolefin shrink films are toys, games, sporting goods, stationery, greeting cards, hardware and household products, office supplies and forms, foods, phonograph records, and industrial parts.
The manufacture of shrink films requires sophisticated equipment including extrusion lines with "orientation" capability, irradiation units when cross-linking is desired, tenter frames, mechanical center-folders, and slitters. "Racking" or "tenter framing" are orientation processes which cause the material to be stretched in the cross or transverse direction and in the longitudinal or machine direction. The films are usually heated to their orientation temperature range which varies with the different polymers but is usually above room temperature and below the polymer's melting temperature. After being stretched, the film is rapidly cooled to quench it thus freezing the molecules of the film in their oriented state. Upon heating, the orientation stresses are released and the film will begin to shrink back to its original unoriented dimension.
The PVC and polyolefin families of shrink films provide a wide range of physical and performance characteristics such as shrink force (the amount of force that a film exerts per unit area of its cross-section during shrinkage), the degree of free shrink (the reduction in surface area a material undergoes when unrestrained), tensile strength (the highest force that can be applied to a unit area of film before it begins to break), sealability, shrink temperature curve (the relationship of shrink to temperature), tear initiation and resistance (the force at which a film will begin to tear and continue to tear), optics (gloss, haze and transparency of material), and dimensional stability (the ability of the film to retain its original dimensions under all types of storage conditions). Film characteristics play an important role in the selection of a particular film and may differ for each type of packaging application and for each packager. Consideration must be given to the product's size, weight, shape, rigidity, number of product components, other packaging materials which may be used along with the film, and the type of packaging equipment available.
Polyolefins have been most successful with applications where moderate to high shrink tensions are preferred; and, on new, automatic, high speed shrink wrapping equipment where shrink and sealing temperature ranges are more closely controlled. The polyolefins tend to be cleaner leaving fewer deposits and less residue thereby extending the life of the equipment and reducing equipment maintenance. The PVC films generally have better optics, lower shrink tensions, and will seal with greater strength at elevated temperatures and shrink over much broader temperature ranges than the polyolefins. Polyolefins usually do not emit corrosive gases upon sealing as do PVC films and, in this respect, are also cleaner than PVC films.
Heretofore, polyolefins have not been able to penetrate PVC film packaging applications where the products to be packaged require the lower shrink tensions of the PVC film because the products are too fragile for use with polyolefins which possess shrink tensions up to four times those of the PVC films. Such products include for example textiles, record jackets, and the like. PVC film is also the shrink film of choice for older, manually operated sealers and semi-automatic wrappers where temperatures are highly variable. Older, poorly maintained wrapping equipment of any type usually runs PVC better than present monolayer polyolefins due to the combination of the generally broader shrink and sealing temperature ranges of the PVC films. In addition, products with sharp or pointed extensions will often require PVC due to the high initial tear resistance of the PVC film relative to that of the polyolefins, i.e. it takes about 7 grams of force to propagate a tear in PVC whereas only 2 to 3.5 grams of force are necessary to propagate a tear in a typical monolayer polyolefin shrink film.
Some success has been achieved by the use of certain multilayer polyolefin shrink films having three or five layer construction and including a core layer and/or outer layers of polyester or copolyester. Exemplary of such films is MPD 1055 manufactured by W. R. Grace & Co. While such shrink films have been successful for many applications, polyester and copolyester are expensive resins. It is therefore desirable to produce a more economical film for shrink applications where fragile products are to be packaged. It would also be advantageous to produce shrink films having improved heat seal strengths, and even lower shrink tensions than the presently available multilayer polyolefin shrink films. Additionally, it is desirable to obtain such a film which nevertheless maintains high free shrink properties at relatively low temperatures. This facilitates effective shrinking at temperatures typically used in commercial heat shrink equipment without the adverse effects on package and product which can result from too high a shrink temperature regime.
Accordingly, it is a general object of the present invention to provide a shrinkable polyolefin film that will have many of the desirable qualities of PVC films and will overcome many of PVC's limitations.
Specifically, it is an object of the present invention to provide a polyolefin film having a combination of improved (i.e. lower) shrink tensions approximating those of PVC films and also providing high free shrink, good optical qualities, a wide shrink temperature range, improved sealability, resistance to tear propagation, and hot seal strength greater than that of prior art multi-layer polyolefin films.
In addition, it is an object of the present invention to provide a polyolefin film which has none of the undesirable qualities of PVC films such as noxious odors and corrosive by-products. These and other objects are achieved by the multilayer polyolefin shrink film which is disclosed herein.